The invention pertains to drag-type drill bits, and, more particularly, to the type of drag bit in which a plurality of cutting members are mounted in a bit body. Such cutting members are formed with a cutting face terminating in a relatively sharp cutting edge for engaging the earth formation to be drilled. In use, the cutting members wear. If the cutting members were formed of a single or uniform material, such wear would occur in a pattern which would cause the original sharp edge to be replaced by a relatively broad flat surface contacting the earth formation over substantially its entire surface area. Such flats are extremely undesirable in that they increase frictional forces, which in turn increases the heat generated and the torque and power requirements.
Accordingly, most such cutting members comprise a stud or similar mounting body formed of one material and carrying a layer of substantially harder material which defines the cutting face. Typically, the mounting body is comprised of cemented tungsten carbide, while the layer defining the cutting face is comprised of polycrystalline diamond or other superhard material. Such use of layers of different materials renders the cutting members "self-sharpening" in the sense that, in use, the member will resist becoming blunt by tending to renew its cutting edge. The tungsten carbide material will tend to wear away more easily than the polycrystalline diamond material. This causes the development of a small step or clearance at the juncture of the two materials so that the earth formation continues to be contacted and cut substantially only by the edge of the diamond layer, the tungsten carbide substrate having little or no high pressure contact with the earth formation. Because the diamond layer is relatively thin, the edge thus maintained is sharp.
It has been found that the effectiveness of such setting members and the bit in which they are employed can be improved by proper arrangement of the cutting members, and more specifically, their cutting faces, with respect to the body of the drill bit, and thus, to the earth formation being cut. The cutting faces are typically planar (although outwardly convex cutting faces are known). The cutting members can be mounted on the bit so that such planar cutting faces have some degree of side rake and/or back rate. Any given drill bit is designed to cut the earth formation to a desired three dimensional "profile" which generally parallels the configuration of the operating end of the drill bit. "Side rake" can be technically defined as the complement of the angle between (1) a given cutting face and (2) a vector in the direction of motion of said cutting face in use, the angle being measured in a plane tangential to the earth formation profile at the closest adjacent point. As a practical matter, a cutting face has some degree of side rake if it is not aligned in a strictly radial direction with respect to the end face of the bit as a whole, but rather, has both radial and tangential components of direction. "Back rake" can be technically defined as the angle between (1) the cutting face and (2) the normal to the earth formation profile at the closest adjacent point, measured in a plane containing the direction of motion of the cutting member, e.g. a plane perpendicular to both the cutting face and the adjacent portion of the earth formation profile (assuming a side rake angle of 0.degree.). If the aforementioned normal falls within the cutting member, then the back rake is negative; if the normal falls outside the cutting member, the back rake is positive. As a practical matter, back rake can be considered a canting of the cutting face with respect to the adjacent portion of the earth formation profile, i.e. "local profile," with the rake being negative if the cutting edge is the trailing edge of the overall cutting face in use and positive if the cutting edge is the leading edge.
Substantial positive back rake angles have seldom, if ever, been used. Thus, in the terminology of the art, a negative back rake angle is often referred to as relatively "large" or "small" in the sense of its absolute value. For example, a back rake angle of -20.degree. would be considered larger than a zero back rake angle, and a back rake angle of -30.degree. would be considered still larger.
Proper selection of the back rake angle is particularly important in adapting a bit and its cutting members for most efficient drilling in a given type of earth formation. In soft formations, relatively small cutting forces may be used so that cutter damage problems are minimized. It thus becomes possible, and indeed preferable, to utilize a relatively small back rake angle, i.e. a very slight negative rake angle, a zero rake angle, or even a slight positive rake angle, since such angles permit fast drilling and optimize specific energy. However, in hard rock, it is necessary to use a relatively large rake angle, i.e. a significant negative rake angle, in order to avoid excessive wear in the form of breakage or chipping of the cutting members due to the higher cutting forces which become necessary.
Problems arise in drilling through stratified formations in which the different strata vary in hardness as well as in drilling through formations which, while substantially comprised of relatively soft material, contain "stringers" of hard rock. In the past, one of the most conservative approaches to this problem was to utilize a substantially negative back rake angle, e.g. -20.degree., for the entire drilling operation. This would ensure that, if or when hard rock was encountered, it would be drilled without damage to the cutting members. However, this approach is unacceptable, particularly where it is known that a substantial portion, and specifically the uppermost portion, of the formation to be drilled is soft, because the substantial negative back rake angle unduly limits the speed of drilling in the soft formation.
Another approach, applicable where the formation is stratified, is to utilize a bit whose cutting members have smaller zero back rake angles to drill through the soft formation and then change bits and drill through the hard formation with a bit whose cutting members have larger back rake angles, e.g. -20.degree. or more. This approach is unsatisfactory because of the time and expense of a special "trip" of the drill string for the purpose of changing bits.
It is believed that the formation is uniformly soft, a somewhat daring approach is to utilize the relatively small back rake angle in order to maximize the penetration rate. However, if a hard stringer is encountered, catastrophic failures can result. For example, severe chipping of only a single cutting member increases the load on neighboring cutting members and shortens their life resulting in a premature "ring out," i.e. a condition in which the bit is effectively inoperative.